I wasn't sure about upgrading to more then 5.1 either, but I tried it, I actually run 6.1 and with the small amount of material that is there with more then 5.1 I find it fun to have those extra channels or channel back there. I find myself looking back there for something that heard that surprised me..
Is it totally needed, no, is it cool and fun.. sure... I like it. I can't do 7.1 in my layout, but that single extra bi-pole speaker sounds awesome when the content is there. I have a weird layout...
Most people do bi-poles as side surround and direct radiator or "bookshelf" as their rear... but either will work, I have seen it both ways. As long as your happy with it. If your really close to the rear wall, you should bring the couch out away, if its too close, and you can't change it, go with bi-pole.
Try it and see what happens... its fun to experiment..
If you don't have side surrounds now, your system is not setup properly... 5.1 is side surrounds, and 7.1 you add speakers to the rear, If Im interpretting your post right here..